The present invention relates to an air-plethysmograph for vascular examination of limbs. More particularly, it pertains to a calibrated apparatus and method for measuring limb volume changes through changes in pressure in a closed pneumatic system.
Reconstructive surgery of deep veins, and a continuing controversy about the mechanism of the effect of elastic compression on venous hemodynamics in the lower limb, have created a need for a noninvasive measurement of reflux and calf muscle pump ejection. Plethysmographs have been used to study limb muscle pump function by determining pressure changes in a cuff surrounding a limb due to postural changes and exercise.
A prior form of plethysmograph, disclosed by Christopoulos et al. in "Air-plethysmography and the effect of elastic compression on venous hemodynamics of the leg", Journal of Vascular Surgery, Vol. 5, No. 1, pg. 148-159, January 1987, consists of an air chamber which surrounds a limb and is inflated to a pressure sufficient to ensure good contact with the limb. Because pressure and volume are inversely related in a closed system, and be the volume of the air chamber is inversel related to the volume of the limb that it surrounds, pressure in the air chamber is directly proportional to the volume of blood in the limb. Problems arise in calibrating the pressure to volume units, however, as attempted by Christopoulos et al., with a second chamber between the air chamber and the limb. The air chamber is initially inflated and closed off from the atmosphere while the limb is kept in an elevated position. After a preselected waiting period, the pressure in the air chamber is measured. A calibration is then performed by injections of known volumes of water into the second chamber. The pressure in the air chamber is measured after each injection, yielding a water volume calibration curve which correlates air chamber pressure with increases in water volume. The water is then removed from the second chamber and vascular examination is conducted. During examination the patient is asked to perform various tests, such as arterial inflow and venous obstruction tests, and the pressure within the air chamber is recorded by a chart recorder. Attempts are made to match recorded pressures with volumes using the volume calibration curve.
Unfortunately, the use of water in the second chamber of Christopoulos et al. complicates the procedure and disrupts the temperature gradient between the air chamber and the limb. This can cause significant output errors. The arbitrary units of the pressure signal also make it difficult to read in terms of volume. Volumes are known at specific points, but interpolation is difficult.
Other devices of this type have a small syringe (.about.1 cc) in communication with an air chamber for injecting a known amount of air into a cuff. The injection of air acts as a reduction in system volume which calibrates the chart for a 1 cc increase in limb volume. However, these devices are calibrated "on line", like the Christopoulos et al. device, and do not produce a one-to-one correspondence between divisions on the chart and units of displayed volume.
Therefore, it is desirable in many applications to provide an apparatus and a method for easily calibrating the output of a plethysmograph to limb volume over a range of volumes and reducing output errors.